1. Field of the Invention
The present invention relates to an active matrix organic light emitting diode (AMOLED) panel, and more particularly, to an AMOLED panel with an adjustable common voltage.
2. Description of Related Art
The current generation is frequently proclaimed as the 3C era: the Computer, the Communication and the Consumer electronics era. In our daily life, we encounter all kinds of information products such as mobile phones, personal digital assistants (PDAs), global positioning satellite (GPS) systems, digital cameras and displays. However, most information equipment uses a flat panel display as the main communication medium.
With the proliferation of information equipment, a variety of flat panel displays, for example, liquid crystal display, plasma display and organic light emitting diode (OLED) panel, are available for selection. The OLED panel not only has the advantages of a higher brightness level, lower power consumption, higher contrast, rapid response and lower driving voltage, but also has the capability to be miniaturized according to the current trend of communication equipment. Therefore, a large number of OLED panel products are developed in recent years.
According to the driving method of the organic light emitting diode (OLED), OLED panels can be classified into passive matrix organic light emitting diode (PMOLED) panels and active matrix organic light emitting diode (AMOLED) panels. Since AMOLED panels can be applied to a large size panel, it has more development potential. In general, the pixel unit of an AMOLED panel at least comprises a transistor and an organic light emitting diode. The transistor has a first terminal connected to a operating voltage and a second terminal connected to the anode of the organic light emitting diode. The cathode of the organic light emitting diode is coupled to a common voltage. In addition, after the AMOLED panel is being operated for a period of time, the voltage difference across the terminals of the organic light emitting diode (OLED) of the AMOLED panel will increase along with the operating time. However, the voltage difference between the first and second terminals of the transistor together with the voltage difference across the terminals of the OLED are a constant value equivalent to the voltage difference between the operating voltage and the common voltage. Hence, if the voltage difference across the OLED is allowed to continuously increase, the voltage difference between the first and second terminal of the transistor will decrease continuously. When the voltage difference changes too much, the transistor can no longer provide a stable driving current to drive the OLED. Ultimately, the picture quality of the AMOLED panel is also adversely affected.